Looking at the future, besides SharePoint having its own App Model, solutions are evolving, they are not limited anymore to what we can do within the platform.

Cloud solutions are becoming interconnected, multi-platform focused, so studying only the SharePoint App Model itself is not enough to master, you need to know Office 365 Development APIs. Hands-on training is available here: https://github.com/OfficeDev/TrainingContent

When you think you have mastered something, you realize you are only on the top of the iceberg. :)